So the desire is to get accurate temperature measurements with a thermister. And that using a external voltage to the AREF pin will keep voltage for the IO pins ratiometric to the external voltage source.
But according to this document, The AREF pin on the Nano 33 BLE is internal only and not connected to anything.
Then according to this document the internal AREF reference for the Nano 33 BLE is 3.3V by default or it can be set to something else.
Does that mean that by default the Nano 33 BLE automatically stabilizes itself to a nice clean 3.3V and and the IO pins will have a high value of 1023 whether the voltage on the 3.3V pin drops to 3.1 volts momentarily?
According to the Arduino product description the Nano 33 BLE processor is a nRF52840. According to the nRF52840 data sheet it doesn't have an external AREF capability and the internal AREF is selectable to be either VDD/4 or 0.6V and you can select an input gain of 1/6 to 4. So, e.g. the full scale input could be VDD with AREF of VDD/4 and a gain of 1/4 or 3V6 with an AREF of 0.6V and a gain of 1/6 or all sorts of other combinations.
So the short answer is no because I don't think the HIGH I/O voltage is proportional to AREF.
No, sorry, I don't have any personal experience with the Nano 33 BLE. It uses an MPM3610 to regulate 5V to 3V3 so you could look at the data sheet for that and get an idea.
If you are still concerned about thermistor current usage when the Nano is powered down, be aware that if you use the 3V3 from the Nano it will go away as soon as you power it down. Don't you have some scheme to power down 60 seconds after the ignition is switched off?
That's the problem. I never power down the Arduino. I leave it on at all times to run the after run code. So I need to leave it running but shut off the 3.3V pin only after the 50 second post run.
Honestly its just waiting for the next time someone comes along and starts it. They make external relays that actually do just that and they also have a parasitic loss as power is on at all times. But me being a hard case I just want to put it all in one package and use the Arduino to do the after run.
I really don't understand what that means. How does the Arduino interact with the someone that comes along? Are you talking about some sort of keyless entry and start or something like that? If you are tired talking about it I understand and will toddle off to other topics.
The Arduino has a car battery input, which is on at all times powering the Arduino. It also a key on input which sets a signal to analog pin A0 through a voltage divider. That pin tell the Arduino the car is running and puts the fan (and hopefully the sensors) to run mode. My cars are old and have keys.
Oh, sorry, I thought the Arduino was powered when the Ignition was turned on. I guess that wouldn't work because it needs to stay powered for the 50 seconds after you kill the engine. I don't think you ever revealed what happens in the after run cycle so I can't comment about how to accomplish that after you power the Arduino down.
I agree that past post 5, this got off topic and conflated with my other thread. But this topic is reason for a new thread and is specific to the Nano 33 BLE in that AREF works differently on all the boards, so it was asked here. EmilyJane offered an answer in post 2 but not much to back it. You had stated in my other thread that you wore not familiar with the Nano 33 BLE and the AREF pin function on it was different.
I appreciate all the help you have given me. But the first post of this thread still remains unanswered to my satisfaction.
If you had asked that if you have the 3V3 powering the Nano measured by an ADC input and you had selected 10-bit resolution for the Nano ADC then if the 3V3 powering the Nano dropped to 3V1 would the ADC reading still be 1023 then I think the answer would be yes.
But I think that would lead you to believe that you could use an IO pin as a reliable excitation for your thermistors and I think the answer to that is no.
If you had asked that if you have the 3V3 powering the Nano measured by an ADC input and you had selected 10-bit resolution for the Nano ADC then if the 3V3 powering the Nano dropped to 3V1 would the ADC reading still be 1023 then I think the answer would be yes.
That's exactly what I am asking, but do you think or do you know. Or can you think of a way to test it.
But I think that would lead you to believe that you could use an IO pin as a reliable excitation for your thermistors and I think the answer to that is no.
If you use the voltage that is powering your Nano to power your thermistors then as was said in the other thread that is a ratiometric application. You could test that by somehow varying the 3V3 on the Nano board but I don't know how you would do that without cutting some traces or some other difficult way. Maybe someone else can answer your question to your satisfaction.